1. Field of the Invention
This invention relates to ferrites and, more particularly, pertains to a new strontium ferrite borate.
2 Description of the Prior Art
As used herein, the term ferrite refers to compounds which can be described as oxides that contain iron (Fe) as a major constituent. Ferrites often have useful magnetic properties. For example, strontium hexaferrite, SrO.6Fe.sub.2 O.sub.3 or SrFe.sub.12 O.sub.19, is an important material used in permanent magnets.
Closely related to strontium hexaferrite are barium hexaferrite and lead hexaferrite. As pointed out by Tokar, these three compounds form a continuous series of substitutional solid solutions. Thus, for example, lead (Pb) can replace strontium (Sr) in any proportion.
Besides permanent magnets, ferrites are used in such diverse applications as microwave and millimeter-wave devices, magneto-optical devices, and computer memories (core memories, magnetic tape, maqnetic discs). Some ferrite applications require that the material be in single crystal form. Other applications can use, or may require, a polycrystalline or ceramic form.
Another kind of ferrite is iron borate, FeBO.sub.3. As pointed out by Andlauer et al, this material is one of only two known compounds which combine optical transparency in the visible spectrum with spontaneous magnetization at room temperature. Therefore, iron borate is of interest for magneto-optical devices.
Polycrystalline or ceramic ferrites are usually prepared by sintering. In this process, high temperatures are used to obtain solid state reactions and densification of mixtures of finely divided powders. The powders are usually oxides of the metallic constituents of the desired ferrite although sources of the oxides may also be used such as, for example, the carbonates or nitrates of the metallic constituents.
Sometimes a small amount of an additional component is added to the mixture of powders as a sintering aide. For example, Tokar has described the use of boron oxide in amounts less than two weight percent as a sintering aide for the preparation of hexagonal ferrites. Part of the function of the boron oxide is to provide a low melting point phase which permits a liquid phase sintering process to operate. The presence of the small amount of liquid phase speeds the reaction and may permit the use of lower sintering temperatures.